Storage permissions endpoint docs (#4319)

* Add docs for storage permisions requests

* add scaladoc

delta/sdk/src/main/scala/ch/epfl/bluebrain/nexus/delta/sdk/permissions/StoragePermissionProvider.scala

@@ -6,6 +6,9 @@ import ch.epfl.bluebrain.nexus.delta.sdk.permissions.model.Permission
 import ch.epfl.bluebrain.nexus.delta.sourcing.model.ProjectRef
 import monix.bio.UIO
 
+/**
+  * Provides the permission a user needs to have in order to access files on this storage
+  */
 trait StoragePermissionProvider {
 
   def permissionFor(id: IdSegmentRef, project: ProjectRef, accessType: AccessType): UIO[Permission]

docs/src/main/paradox/docs/delta/api/user-permissions-api.md

@@ -11,20 +11,31 @@ The described endpoints are experimental and the responses structure might chang
 
 @@@
 
-## Head
+Requests
+: All requests should have no body
 
-This operation determines whether the current logged in user has a specific permission in a specific context
+Responses
+: A response will have a 204 (no content) status code if the user is authorised
+: A response will have a 403 (forbidden) status code if the user is not authorised
+
+
+## Standard permissions
 
+This operation determines whether the current logged in user has a specific permission in a specific context
 ```
 HEAD /v1/user/permissions/{org_label}/{project_label}?permission={permission}
 ```
-
 where
 - `{permission}`: String - the permission to check
 
-Request
-: The request should have no body
 
-Response
-: The response will have a 204 (no content) status code if the user is authorised
-: The response will have a 403 (forbidden) status code if the user is not authorised
+## Storage access permissions
+
+This operation determines whether the current logged in user would be able to access files on a specific storage
+```
+HEAD /v1/user/permissions/{org_label}/{project_label}?storage={storage_id}&type={access_type}
+```
+where
+- `{storage_id}`: String - the id of the storage
+- `{access_type}`: String - the access type of the storage. Can be `read` or `write`
+